High Resolution Stimulated Brillouin Gain Spectroscopy

Brillouin spectroscopy has been an important source of information about the collective dynamics of molecules in liquids.1,2 In a conventional Brillouin experiment, a Fabry-Perot interferometer is used to measure the spectrum of laser light scattered at an angle θ by spontaneous density fluctuations in a sample. The Brillouin peaks in the spectrum arise from the acoustic wave component of these density fluctuations, and accordingly the shift of the Brillouin peaks from the elastic Rayleigh scattering peak corresponds to the acoustic frequency, while the width of the Brillouin peaks corresponds to the acoustic damping rate. By varying θ, and thus the scattering wavevector, one can determine the dispersion in the speed of sound and the acoustic attenuation, which in turn characterize the elastic and viscous responses of the fluid. A more detailed analysis of the spectral lineshape with the aid of generalized hydrodynamic theories allows one to determine the values of transport coefficients that describe the molecular dynamics.1